I. Field of the Invention.
The present invention relates generally to methods and devices for the non-contact monitoring and measurement of the torsional dynamics of rotating shafts. The present invention relates more specifically to methods and devices for the non-contact monitoring and measurement of stationary and transient torques in rotating shafts using magnetostrictive sensors (MsS).
II. Description of the Prior Art.
It is common in many different machine systems, for mechanical power from an electrical motor, combustion engine, or gas turbine to be transmitted to a load through a power train of some type. Rotating shafts are frequently integral parts of such power trains. A variety of machinery dynamics, including vibration, lateral movement, and torsional motion, have a direct effect on the operational conditions of these machine systems. The monitoring of these machinery dynamics offers, therefore, a valuable means of diagnosing and correcting machinery problems in a manner that can assist in the effective operation and maintenance of the machinery.
For example, the measurement of the torsional dynamics of a rotating shaft can be used to control backlash at gear teeth and other types of drive train couplings, for a more efficient operation of the machinery and less wear on the machinery components.
At present, various methods are used to measure the torsional dynamics of a rotating shaft. These methods include shaft encoders, torsional accelerometers, and strain gauges. In general, the currently available methods require physical contact of some type with the rotating shaft (side or end surfaces) and a means of electrical/electronic communication such as slip rings or telemetry for relaying sensor information to signal analysis equipment. In many situations, these methods are not only difficult to use but costly to implement. In addition, the available methods generally lack long-term durability, which is essential for on-line monitoring and control during the service life of high speed rotational machinery.
Some efforts in the past have attempted to implement a non-contact means for the measurement of dynamic torsion in rotating shafts using magnetostrictive techniques. None of these efforts, however, disclose or anticipate a detector that does not include some form of periodic external excitation of the magnetostrictive material. The following patents are considered illustrative of the art encountered within the field.
U.S. Pat. No. 4,979,399, issued to Klauber et al. on Dec. 25, 1990, entitled “Signal Dividing Magnetostrictive Torque Sensor”, describes a non-contacting method for sensing torque utilizing the magnetostrictive principle by inducing a primary magnetic flux in a rotating shaft with an excitation coil.
U.S. Pat. No. 4,939,937, issued to Klauber et al. on Jul. 10, 1990, entitled “Magnetostrictive Torque Sensor”, likewise describes a system for sensing torque based on the magnetostrictive principle that utilizes a primary excitation coil to introduce a magnetic flux in the rotating shaft. The system involves appropriate placement both of a sensor coil and the primary excitation coil in positions adjacent to the rotating shaft and appropriate orientation of the coils with respect to each other.
U.S. Pat. No. 4,811,609, issued to Nishibe et al. on Mar. 14, 1989, entitled “Torque Detecting Apparatus”, describes a system for measuring the transmitted torque within a rotating magnetic material by means of a magnetostrictive sensor. Essential to the Nishibe system is the use of a demagnetization coil designed to restore the rotary magnetic material to a state of zero magnetization. Included in this system are driving circuits and excitation coils for establishing the magnetic field within the rotating shaft.
U.S. Pat. No. 4,803,885, issued to Nonomura et al. on Feb. 14, 1989, entitled “Torque Measuring Apparatus”, also describes a non-contact method for measuring torque in a rotating shaft of ferromagnetic material using magnetic based sensors. The device includes an excitation coil wound around the outer periphery of the rotating shaft and adapted to magnetize the shaft in an axial direction. A detecting core ring in the form of an integral unit includes a number of detecting cores arranged around the circumferential area of the rotating shaft.
U.S. Pat. No. 3,046,781, issued to Pratt on Jul. 31, 1962, entitled “Magnetostrictive Torque Meter”, provides an early teaching of the basic approach of employing magnetostrictive principles to implement a torque meter for a rotating shaft based on stress measurements of the shaft material. The description of the operation of the non-contacting embodiment (shown in FIG. 1 of the Pratt patent) refers to the use of an AC excitation coil wound around the shaft, and the inclusion of a “magnetostriction constant”.
Japanese Patent No. 3-269228, issued to Aisin Seiki on Nov. 29, 1991, entitled “Magnetostriction Detector for Torque Detector Using Film of Magnetostrictive Metal Containing Super Magnetostrictive Alloy Particles”, describes a system for measuring torque in a rotating shaft utilizing a primary excitation coil and a secondary detection coil adjacent to a surface on the shaft that has been covered with a ferromagnetic material. The focus of this patent involves the type of metallic material utilized as the magnetostrictive substance.
Each of the above patents describe devices for measuring torque in a shaft using a similar approach that requires a means for applying an AC magnetic field to the ferromagnetic shaft material. Most of the later issued U.S. patents provide teachings of similar magnetostrictive torque measuring approaches. Some of these patents suggest using a thin coating of magnetostrictive materials around non-ferromagnetic materials as is well known in the field of magnetostrictive sensing.
III. Background on the Magnetostrictive Effect.
The magnetostrictive effect is a property peculiar to ferromagnetic materials. The magnetostrictive effect refers to the phenomena of physical, dimensional change associated with variations in magnetization. The effect is widely used to make vibrating elements for such things as sonar transducers, hydrophones, and magnetostrictive delay lines for electric signals. The magnetostrictive effect actually describes physical/magnetic interactions that can occur in two directions. The Villari effect occurs when stress waves or mechanical waves within a ferromagnetic material cause abrupt, local dimensional changes in the material which, when they occur within an established magnetic field, can generate a magnetic flux change detectible by a receiving coil in the vicinity. The Joule effect, being the reverse of the Villari effect, occurs when a changing magnetic flux induces a mechanical vibrational motion in a ferromagnetic material through the generation of a mechanical wave or stress wave. Typically, the Joule effect is achieved by passing a current of varying magnitude through a coil placed within a static magnetic field thereby modifying the magnetic field and imparting mechanical waves into a ferromagnetic material present in that field. These mechanical or stress waves then propagate not only through the portion of the ferromagnetic material adjacent to the generating coil but also into and through any further materials in mechanical contact with the ferromagnetic material. In this way, non-ferromagnetic materials can serve as conduits for the mechanical waves or stress waves that can thereafter be measured by directing them through these ferromagnetic “wave guides” placed proximate to the magnetostrictive sensor element.
The advantages of magnetostrictive sensors over other types of vibrational sensors becomes quite clear when the structure of such sensors is described. All of the components typically utilized in magnetostrictive sensors are temperature, pressure, and environment-resistant in ways that many other types of sensors, such as piezoelectric based sensors, are not. High temperature, permanent magnets, magnetic coils, and ferromagnetic materials are quite easy to produce in a variety of configurations. Further, although evidence from the previous applications of magnetostrictive sensors would indicate the contrary, magnetostrictive sensors are capable of detecting mechanical waves and translating them into signals that are subject to very fine analysis and discrimination in a manner that allows information to be obtained about the elements in an object (such as a rotating steel shaft) that may have initially generated the stress.
It would be desirable, therefore, to have a torque measurement system that utilizes magnetostrictive sensors in conjunction with a rotating shaft. It would be desirable to maintain the advantages of such a system through its non-contact method of detecting the magnetostrictive effect within ferromagnetic material contained on or in the rotating shaft. In addition, it would be preferable to simplify such a system by eliminating the need for at least the primary excitation coil found in each of the existing systems based on magnetostrictive sensors. It would be desirable to implement such a system with a magnetostrictive sensor that provides a signal which, when amplified and appropriately filtered, carries the same information about the torque being experienced in the rotating shaft as more expensive, cumbersome, and delicate systems that use strain transducers, telemetry devices and the like.